Process for preparing expandable particles of vinyl chloride resin composition

ABSTRACT

A process for preparing expandable particles of a vinyl chloride resin composition suitable for beads-foaming, comprising steps of: 
     melting 100 parts by weight of a vinyl chloride resin having a difference between T 1  and T 2  of not higher than 45° C. with 1 to 30 parts by weight of an acrylic resin to form a gelled composition, 
     pelletizing said composition, and 
     impregnating the resulting pellets with an easily volatile organic blowing agent and an organic compound capable of dissolving or swelling said vinyl chloride resin; said T 1  and T 2  being measured by a Koka Shiki Flow A-method under conditions of 1 mm in nozzle diameter, 1 mm in nozzle length, 6° C./minute in temperature rise rate and 100 kg/cm 2  in load. By the process of the present invention, expandable particles moldable by beads-foaming to foamed articles having desired complicated shapes with smooth surfaces and high expansion ratios can be easily prepared.

BACKGROUND OF THE INVENTION

The present invention relates to a process for preparing expandable particles of a vinyl chloride resin composition usable for beads-foaming.

As processes for preparing foamed articles of a vinyl chloride resin composition, there have hitherto been known (a) a process in which a chemical blowing agent which gasifies by heat decomposition is admixed with a vinyl chloride resin and the mixture is heat-molded to foamed articles by an extruder or an injection molding machine; (b) a process in which a vinyl chloride resin and a plasticizer are mixed to prepare a plastisol, and the plastisol is mechanically foamed, or a chemical blowing agent is admixed with the plastisol and the mixture is melted and foamed by heating; (c) a process in which a mixture of a vinyl chloride resin and a chemical blowing agent is previously roll-molded at a temperature of not higher than the decomposition temperature of the chemical blowing agent and then foamed by heating; (d) a process in which a mold is filled with a vinyl chloride resin and a chemical blowing agent (if necessary, an easily volatile organic blowing agent and an organic solvent which swells or softens the resin are additionally used), heated under pressure to melt, cooled down, and heated again to foam; (e) a process in which an extruder is charged with a vinyl chloride resin into which a volatile blowing agent is previously impregnated or an extruder is charged with a vinyl chloride resin and then a blowing agent is introduced into the extruder to carry out extrusion foaming; and the like.

However, the above-mentioned processes (a) to (e) have the following various defects.

By the processes (a) to (c), rigid or semi-rigid foamed articles having high expansion ratios cannot be prepared; as to the process (d), the costs of end products are great since the process is a batchwise process of which producing steps are complicated and a lot of time is required for preparing the foamed articles; and as to the process (e), though rigid or semirigid foamed articles having high expansion ratios can be relatively easily prepared, foamed articles having complicated shapes cannot be prepared due to extrusion foaming.

Vinyl chloride resin foamed articles have various advantages such as good noninflammability, good toughness and low cost over polystyrene foamed articles. However, there have not been provided expandable particles of vinyl chloride resins which can be molded in high expansion ratios by beads-foaming technique.

In view of the above-mentioned defects of the conventional processes for preparing foamed articles of a vinyl chloride resin composition and in order to prepare easily foamed articles having complicated shapes, the present inventors made a study of a process in which melting and pelletization of a specific vinyl chloride resin were carried out, the pellets were impregnated with a volatile blowing agent and heated to make pre-expanded particles, and the pre-expanded particles were molded in a mold. By this process, though pre-expanded particles were prepared, foamed articles having desired shapes could not be made by heat-molding in a mold. That is, when the pre-expanded particles were molded at a low temperature, they did not fuse together and therefore the molded article broke when it was taken out from the mold, and when the pre-expanded particles were molded at a high temperature, they melted and returned to their original pellets. After a further in-depth study, the present inventors have found that foamed articles having desired shapes can be prepared only when specific resin compositions and specific blowing agents are combined.

An object of the present invention is to provide a process for preparing expandable particles of a vinyl chloride resin composition which can make foamed articles having desired shapes.

The above and other objects of the present invention will become apparent from the description hereinafter.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a process for preparing expandable particles of a vinyl chloride resin composition suitable for beads-foaming, comprising the steps of:

melting 100 parts by weight of a vinyl chloride resin having a difference between T₁ and T₂ of not higher than 45° C. with 1 to 30 parts by weight of an acrylic resin to form a gelled composition,

pelletizing said composition, and

impregnating the resulting pellets with an easily volatile organic blowing agent and an organic compound capable of dissolving or swelling said vinyl chloride resin; T₁ and T₂ being measured by a Koka Shiki Flow Amethod under conditions of 1 mm in nozzle diameter, 1 mm in nozzle length, 6° C./minute in temperature rise rate and 100 kg/cm² in load.

DETAILED DESCRIPTION

It is necessary that the vinyl chloride resins used in the present invention are those having differences between T₁ and T₂ of not higher than 45° C. T₁ and T₂ are measured by means of the Koka Shiki Flow A-method provided by Kobunshi Kagaku Gakkai in Japan under the conditions of 1 mm in nozzle diameter, 1 mm in nozzle length, 6° C./minute in temperature rise rate and 100 kg/cm² in load (refer to Japanese Industrial Standard K7210 "Flow Testing Method (Reference Test)" with respect to the detail of the apparatus used). T₁ is a softening point of a resin and T₂ is a temperature at which the resin begins to flow out from the nozzle when a load is applied to the resin by a plunger. When a vinyl chloride resin having the difference between T₁ and T₂ of higher than 45° C. is used, expansion ratios because the resin is extremely poor in expandability. From such a viewpoint, as a preferable vinyl chloride resin used in the invention, there are exemplified a copolymer of vinyl chloride and vinyl acetate, e.g. a vinyl chloride/vinyl acetate copolymer having an average degree of polymerization of 300 to 3,500 and containing 5 to 20 % by weight of vinyl acetate, copolymers of vinyl chloride and various monomers copolymerizable with vinyl chloride such as ethylene and propylene, and the like.

In the present invention, the above specified vinyl chloride resin is used in combination with the acrylic resin. The use of the acrylic resin is required in order to suitably control the melt viscosity of expandable particles of a vinyl chloride resin composition of the invention upon foaming, and to increase cell membrane toughness, thereby increasing expandability of the particles upon pre-expanding and moldability of the pre-expanded particles in a mold. As an acrylic resin satisfying the above-mentioned objects, copolymers containing methyl methacrylate as a main component are preferable. Examples of the copolymers are, for instance, a copolymer of methyl methacrylate and an acrylic ester, a copolymer of methyl methacrylate, an acrylic ester and a monomer copolymerizable therewith, and the like. When the content of methyl methacrylate in the acrylic resin is high, the expansion ratio becomes high because elongation of the resin composition increases at a high molding temperature. Accordingly, it is preferable that the content of methyl methacrylate is not lower than 50 % by weight.

Examples of the acrylic esters are, for instance, methyl acrylate, ethyl acrylate, n-butyl acrylate, i-butyl acrylate, 2-ethylhexyl acrylate, and the like. Examples of the copolymerizable monomers are, for instance, styrene, an unsaturated nitrile, a vinyl ester, a methacrylic ester other than methyl methacrylate such as ethyl methacrylate, n-butyl methacrylate and 2ethylhexyl methacrylate, and the like.

It is desirable that the amount of the acrylic resin is from 1 to 30 parts by weight based on 100 parts by weight of the vinyl chloride resin, preferably from 5 to 20 parts by weight. When the amount is smaller than 1 part by weight, the above-mentioned effects cannot be produced. The use of higher than 30 parts by weight of the acrylic resin is not necessary because no further effects are produced on expansion and rather lowers noninflammability originating from the vinyl chloride resin.

The acrylic resin and the vinyl chloride resin are melted, mixed and pelletized. The resin composition of the vinyl chloride resin and the acrylic resin may contain usual additives such as stabilizer, lubricant, plasticizer, filler and coloring agent. Known processes for such melting, mixing and pelletizing can be adopted in the invention. For example, after a mixture of the acrylic resin powder, the vinyl chloride resin powder, a stabilizer, and various additives such as a lubricant are melted by heating in an extruder and extruded through holes of a die, and the extrudate is cut by a pelletizer; or after preparing sheets of a molten mixture of the acrylic resin and the vinyl chloride resin by a heat-roll, the sheets are cut into pellets. In addition to the above-mentioned methods, there are many methods variously combining different melting steps and different pelletization steps. Also, pellets can be prepared by suspension polymerization of vinyl chloride monomer in the presence of the acrylic resin.

The thus-prepared pellets of the vinyl chloride resin composition are impregnated with a volatile blowing agent in order to give expandability to the pellets. Various volatile orgnaic compounds can be employed in the present invention as the volatile blowing agents. Examples of the volatile blowing agent to be impregnated into the pellets are, for instance, aliphatic hydrocarbons such as propane, n-butane, i-butane, n-pentane, i-pentane, neopentane and n-hexane, halogenated aliphatic hydrocarbons such as dichlorodifluoromethane, trichloromonofluoromethane, dichloromonofluoroethane, trichlorotrifluoroethane and dichlorotetrafluoroethane, and the like. The volatile blowing agents may be used alone or in admixture thereof.

The volatile blowing agent can be impregnated into the pellets in various manners. There are, for example, a method in which pellets of the vinyl chloride resin composition are suspended in water in an autoclave equipped with a stirrer and a required amount of the volatile blowing agent is added to the slurry with stirring, whereby the volatile blowing agent is impregnated into the pellets; a method in which the pellets are exposed to the volatile blowing agent in the form of gas or liquid; and the like.

The above-mentioned volatile blowing agents are poor in compatibility with the vinyl chloride resin, and therefore their impregnation into the pellets is not satisfactory. Accordingly, for impregnating the pellets with a practical amount of the volatile blowing agent, it is preferable to use organic compounds capable of dissolving or swelling the vinyl chloride resin composition along with the volatile blowing agent. The organic compounds can be easily impregnated into pellets of the vinyl chloride resin composition and they accelerate impregnation of the volatile blowing agent.

Examples of the organic compounds are, for instance, halogenated hydrocarbons capable of dissolving or swelling the vinyl chloride resin, such as methyl chloride, methylene chloride, chloroform, carbon tetrachloride, ethyl chloride, ethylidene chloride and tricloroethane, tetrahydrofuran, methyl ethyl ketone, and the like. The organic compounds may be used alone or in a mixture thereof.

The amount of the volatile blowing agent and the organic compound depend on desired expansion ratios. The practical amounts of the former and the latter to prepare foamed articles having a degree of expansion of 2 to 80 cc/g are from 2 to 40 parts by weight and from 1 to 30 parts by weight, preferably from 1 to 15 parts by weight, respectively based on 100 parts by weight of the vinyl chloride resin composition.

The thus-prepared expandable vinyl chloride resin composition particles in the present invention are aged by storing for 2 to 7 days after impregnation of the blowing agent. The aged particles have excellent expandability. The aged particles can be easily molded to make shaped foamed articles as follows:

The expandable particles are expanded by heating with a heatig medium such as steam and hot air to make pre-expanded particles having a desired expansion ratio, and the pre-expanded particles are filled in a mold which is able to be closed, but unable to be sealed and heated with a heating medium such as steam and hot air to make foamed articles of a vinyl chloride resin composition having the same shape as the mold.

The present invention is more specifically described and explained by means of the following Examples in which all percents and parts are by weight unless otherwise noted. It is to be understood that the present invention is not limited to the Examples, and various changes and modifications may be made in the invention without departing from the spirit and scope thereof.

EXAMPLES 1 to 15

The acrylic resins used in Examples are shown in Table 1.

                  TABLE 1                                                          ______________________________________                                         Content                                                                        of        Components          Reduced                                          MMA*      other than MMA      viscosity                                        No.  (%)            Kind     Content (%)                                                                             dl/g (25° C.)                     ______________________________________                                         A-1  93             Butyl     7       12                                                           acrylate                                                   A-2  90             Ethyl    10       10                                                           acrylate                                                   A-3  87             Ethyl    13        4                                                           acrylate                                                   A-4  83             Butyl    13       13                                                           acrylate                                                   A-5  88             Butyl     5        4                                                           methacrylate                                                                   Butyl     7                                                                    acrylate                                                   A-6  60             Butyl    40       10                                                           acrylate                                                   A-7  50             Butyl    50       10                                                           acrylate                                                   A-8  40             Butyl    60       10                                                           acrylate                                                   ______________________________________                                          Note                                                                            *Methyl methacrylate                                                    

A mixture of 100 parts vinyl chloride-vinyl acetate copolymer powder (vinyl acetate content: 13 %, average degree of polmerization: 430, difference between T₁ and T₂ : 24° C., which was measured by means of the Koka Shiki Flow A-method), the acrylic resins which are shown in Table 1 and of which amounts of use are shown in Table 2, 2 parts of dioctyltin maleate as a stabilizer and 0.5 part of fatty ester lubricant (montanic acid ester) was kneaded in an extruder at 180° C. and extruded to make pellets having a diameter of 1.0 mm and a length of 1.5 mm.

In an autoclave equipped with a stirrer, 800 g of the pellets were suspended in 1,200 ml of water containing 1 % of calcium tertiary phosphate and 0.0025 % of sodium alkylbenzenesulfonate (sodium dodecylbenzene-sulfonate) to make a slurry. There were added 120 g of butane and 60 g of ethylene chloride to the slurry with stirring, and the mixture was maintained at 110° C. for 7 hours, cooled to a room temperature and taken out. The prepared expandable particles were stored in a refrigerator at 10° C. for 7 days, and then the aged particles were heated for 1 minute by steam of which temperature is 100° C. to make pre-expanded particles.

The pre-expanded particles were filled in a mold with slits (internal sizes: 30-cm length, 9-cm width, 3-cm thickness) and were heated for 1 minute by steam of which temperature is 105° C. and cooled to 60° C. to make a foamed article of the vinyl chloride resin composition.

The molded foamed article was white and had closed cells which were uniform and fine.

The results are shown in Table 2.

COMPARATIVE EXAMPLE 1

The procedure of Example 1 was repeated except that an acrylic resin was not used. The particles did not fuse together and a foamed article could not be made.

The result is shown in Table 2.

                  TABLE 2                                                          ______________________________________                                         Acrylic resin                Surface                                           Kind     Amount   Density (g/cm.sup.3)                                                                          smoothness*                                   ______________________________________                                         Ex. 1 A-1     5       0.033        from  ○  to ⊚         2     A-1    10       0.025        ⊚                            3     A-1    15       0.023        ⊚                            4     A-1    20       0.023        ⊚                            5     A-2    10       0.028        ⊚                            6     A-3    10       0.030        ⊚                            7     A-4    10       0.032        ⊚                            8     A-5    10       0.028        ⊚                            9     A-6    10       0.040        ○                                    10    A-6    15       0.035        ○                                    11    A-6    20       0.033        ○                                    12    A-7    20       0.040        ○                                    13    A-1     1       0.038        Δ                                     14    A-8    10       0.063        Δ                                     15    A-8    20       0.058        Δ                                     Com.  --      0       A foamed article                                                                            --                                          Ex. 1                 could not be obtained.                                   ______________________________________                                          (Note)                                                                         *The surface smoothness of obtained foamed article is observed with the        naked eye.                                                                     (Estimation)                                                                   ⊚: Excellent                                                    ○: Good                                                                 Δ: Poor                                                            

From the Table 2, it is understood that foamed articles having excellent appearance can be prepared from the expandable particles of the present invention. Particularly, when acrylic resins containing not smaller than 50 % of MMA are used, the effect of the invention is remarkable. On the other hand, when an acrylic resin is not used, the particles do not fuse together and foamed articles cannot be prepared.

EXAMPLES 16 to 18 and COMPARATIVE EXAMPLES 2 and 3

The procedure of Example 2 was repeated except that the vinyl chloride resins shown in Table 3 were used.

The results are shown in Table 3.

                                      TABLE 3                                      __________________________________________________________________________     Vinyl chloride resin                                                           Component other than                                                                          Degree of                                                                            Difference                                                vinyl chloride polymeri-                                                                            between Expand-                                                                             Mold-                                                                              Density                                  Kind    Content (%)                                                                           zation (.sup.--P)                                                                    T.sub.1 and T.sub.2 (°C.)                                                       ability                                                                             ability                                                                            (g/cm.sup.3)                             __________________________________________________________________________     Ex. 2                                                                              Vinyl                                                                              13     430   24      ⊚                                                                    ⊚                                                                   0.025                                        acetate                                                                    Ex. 16                                                                             Vinyl                                                                              10     780   40      ○                                                                            ○                                                                           0.052                                        acetate                                                                    Ex. 17                                                                             --  0      450   39      ○                                                                            ○                                                                           0.055                                    Ex. 18                                                                             Ethylene                                                                           5      450   25      ⊚                                                                    ⊚                                                                   0.030                                    Com.                                                                               --  0      780   61      X    *1                                           Ex. 2                                                                          Com.                                                                               Vinyl                                                                              3      780   52      Δ                                                                             *2                                           Ex. 3                                                                              acetate                                                                    __________________________________________________________________________      (Note)                                                                         *1: The particles could not expand and a foamed article could not be made      *2: The particles could scarcely expand and a foamed article could not be      made.                                                                    

[Expandability]

After heating particles in a steamer at 100° C. until they expand to be utmost, sizes of the particles are measured.

The expansion ratio (e) is calculated by the following equation. ##EQU1## wherein a is a size of the original particles and b is a size of the expanded particles.

(Estimation)

⊚: Excellent expansion

○: Good expansion

Δ: Poor expansion

: No expansion

[Moldability]

A mold with slits (length: 30-cm, width: 7-cm, thickness: 3-cm) is filled with pre-expanded particles, heated in an autoclave for 30 minutes by steam under a pressure of 0.5 kg/cm² and cooled in water. The foamed article is taken out from the mold and the surface of the article is observed with the naked eye.

(Estimation)

⊚: Excellent

○: Good

EXAMPLES 19 to 22

The procedure of Example 2 was repeated except that the blowing agents and the organic compounds shown in Table 4 were used.

The results are shown in Table 4.

COMPARATIVE EXAMPLES 4 and 5

The procedure of Example 2 was repeated except that the organic compound was not used.

The results are shown in Table 4.

                                      TABLE 4                                      __________________________________________________________________________     Blowing agent  Organic compound                                                          Amount        Amount                                                                              Expand-                                                                             Mold-                                                                              Density                                  Kind      (part)                                                                              Kind     (part)                                                                              ability                                                                             ability                                                                            (g/cm.sup.3)                             __________________________________________________________________________     Ex. 2                                                                              Butane                                                                               15   Methylene                                                                               7.5  ⊚                                                                    ⊚                                                                   0.025                                                   chloride                                                        Ex. 19                                                                             Butane                                                                               10   Methylene                                                                               5.0  ⊚                                                                    ⊚                                                                   0.028                                                   chloride                                                        Ex. 20                                                                             Butane                                                                               10   Tetrahydrofuran                                                                         7.5  ⊚                                                                    ⊚                                                                   0.031                                    Ex. 21                                                                             Flon-12*.sup.1                                                                       20   Methylene                                                                               7.5  ⊚                                                                    ⊚                                                                   0.026                                                   chloride                                                        Ex. 22                                                                             Flon-114*.sup.2                                                                      20   Methylene                                                                               7.5  ⊚                                                                    ⊚                                                                   0.027                                                   chloride                                                        Com.                                                                               Butane                                                                               15   --       --   --   X   *.sup.3                                  Ex. 4                                                                          Com.                                                                               Flon-12                                                                              30   --       --   --   X   *.sup.3                                  Ex. 5                                                                          __________________________________________________________________________      (Note)                                                                         *.sup.1 Dichlorodifluoromethane                                                *.sup.2 Dichlorotetrafluoroethane                                              *.sup.3 The particles could not expand since the blowing agent could not       be impregnated into the pellets.                                          

What we claim is:
 1. A process for preparing expandable particles of a vinyl chloride resin composition suitable for beads-foaming, comprising steps of:melting 100 parts by weight of a vinyl chloride resin having a difference between T₁ and T₂ of not higher than 45° C. with 1 to 30 parts by weight of an acrylic resin to form a gelled composition, pelletizing said composition, and impregnating the resulting pellets with an easily volatile organic blowing agent and an organic compound capable of dissolving or swelling said vinyl chloride resin; said T₁ and T₂ being measured by a Koka Shiki Flow A-method under conditions of 1 mm in nozzle diameter, 1 mm in nozzle length, 6° C./minute in temperature rise rate and 100 kg/cm² in load.
 2. The process of claim 1, wherein said vinyl chloride resin is a vinyl chloride/vinyl acetate copolymer having an average degree of polymerization of 300 to 3,500 and a vinyl acetate content of 5 to 20 % by weight.
 3. The process of claim 1, wherein said acrylic resin contains not less than 50 % by weight of methyl methacrylate.
 4. The process of claim 1, wherein said acrylic resin is used in an amount of 5 to 20 parts by weight. 